London, Dec 1 : Scientists have determined that due to a dramatic loss of sea-ice in the Arctic during the summer of 2007, convective mixing in the North Atlantic Ocean, a mechanism that helps to remove carbon dioxide (CO2) from the atmosphere, has returned after a decade of near stagnation.

Convective mixing, or 'overturning', of ocean waters at high latitudes helps to drive the Atlantic 'heat conveyor belt' that carries warm water northwards and cooler deep-water back south.

The phenomenon also helps to remove carbon dioxide from the atmosphere. As cold water sinks, it carries dissolved CO2 with it, locking it away in the depths of the ocean for centuries.

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But, cold winters are one of the prerequisites for convection in the deep-water formation regions around Greenland, such as the Labrador and Irminger Seas, and a warming climate would be expected to slow or stop the process.

There's been very little convection in the North Atlantic over the past decade, prompting concerns that the impact of global warming was already being felt.

Now, according to a report in Nature News, two teams of scientists have independently found evidence that overturning has resumed in the North Atlantic.

A team led by Kjetil Vage of the Woods Hole Oceanographic Institution in Massachusetts has found that convection returned to the region last winter (2007-2008).

Igor Yashayaev and John Loder of the Bedford Institute of Oceanography in Nova Scotia, Canada, separately report that the convection last winter in the Labrador Sea was the deepest since 1942.

According to Vage and his colleagues, weather peculiarities alone cannot explain the unexpected return of convection.

Temperatures last winter were 5-6 degrees Celsius colder in the North Atlantic than in the previous seven years. But, the location of high and low pressure systems over the region means that weather patterns did not favour overturning.

The researchers suggest that a cap of cooler fresh water - massive sea-ice export from the Arctic basin along both sides of Greenland during the previous summer - has facilitated freezing of parts of the Labrador and Irminger Seas.

As a result, cold continental-origin air blowing over the region had not been warmed by the relatively warm ocean when it reached the convection areas.

The temperature difference between air and open water led to a massive transfer of heat from the ocean to the atmosphere, thus fuelling convection.